Fix GPU Particles

The special case atan(y,0) of the built-in shader function atan(y,x)
returns different results on different devices. So this commit will add
checks when the atan(y,x) function is used in ParticlesMaterial to set
the direction of GPU Particles to make sure the desired values are
returned (act as atan2(y,x)).

scene/resources/particles_material.cpp

@@ -316,14 +316,17 @@ void ParticlesMaterial::_update_shader() {
 
 	if (flags[FLAG_DISABLE_Z]) {
 
-		code += "		float angle1_rad = atan(direction.y, direction.x) + rand_from_seed_m1_p1(alt_seed) * spread_rad;\n";
+		code += "		float angle1_rad = rand_from_seed_m1_p1(alt_seed) * spread_rad;\n";
+		code += "		angle1_rad += direction.x != 0.0 ? atan(direction.y, direction.x) : sign(direction.y) * (pi / 2.0);\n";
 		code += "		vec3 rot = vec3(cos(angle1_rad), sin(angle1_rad), 0.0);\n";
 		code += "		VELOCITY = rot * initial_linear_velocity * mix(1.0, rand_from_seed(alt_seed), initial_linear_velocity_random);\n";
 
 	} else {
 		//initiate velocity spread in 3D
-		code += "		float angle1_rad = atan(direction.x, direction.z) + rand_from_seed_m1_p1(alt_seed) * spread_rad;\n";
-		code += "		float angle2_rad = atan(direction.y, abs(direction.z)) + rand_from_seed_m1_p1(alt_seed) * spread_rad * (1.0 - flatness);\n";
+		code += "		float angle1_rad = rand_from_seed_m1_p1(alt_seed) * spread_rad;\n";
+		code += "		float angle2_rad = rand_from_seed_m1_p1(alt_seed) * spread_rad * (1.0 - flatness);\n";
+		code += "		angle1_rad += direction.z != 0.0 ? atan(direction.x, direction.z) : sign(direction.x) * (pi / 2.0);\n";
+		code += "		angle2_rad += direction.z != 0.0 ? atan(direction.y, abs(direction.z)) : (direction.x != 0.0 ? atan(direction.y, abs(direction.x)) : sign(direction.y) * (pi / 2.0));\n";
 		code += "		vec3 direction_xz = vec3(sin(angle1_rad), 0.0, cos(angle1_rad));\n";
 		code += "		vec3 direction_yz = vec3(0.0, sin(angle2_rad), cos(angle2_rad));\n";
 		code += "		direction_yz.z = direction_yz.z / max(0.0001,sqrt(abs(direction_yz.z))); // better uniform distribution\n";